There is a storage system as one example of a computer system. As a storage system having a data amount reduction function, for example, a storage system disclosed in U.S. Pat. No. 8,527,544, in other words, a storage system having a garbage collection function is known. In U.S. Pat. No. 8,527,544, the following is disclosed.
A storage system includes a memory device and a storage controller having a garbage collection function. The storage controller manages mapping between a virtual address and a physical address of the memory device. When write for a virtual address occurs, the storage controller performs mapping update (update of an address translation table) of mapping a free physical address of a new write destination for the virtual address instead of a physical address that has already been allocated thereto. As a result, invalid data (data according to a physical address of which mapping for a virtual address has been released) increases. The storage controller executes garbage collection of copying valid data (data according to each physical address mapping into one virtual address) and deleting invalid data. Accordingly, the amount of data is reduced, and the number of free physical addresses (free area) increases.